Default to empty string; Scrutinizer warns us to do additional type check now, but the default value is never returned anyway

Author: tvdijen

src/Backend/OpenSSL.php

@@ -80,7 +80,7 @@ public function encrypt(AbstractKey $key, string $plaintext): string
                 $fn = 'openssl_private_encrypt';
             }
 
-            $ciphertext = null;
+            $ciphertext = '';
             if (!$fn($plaintext, $ciphertext, $key->get(), $this->padding)) {
                 throw new RuntimeException('Cannot encrypt data: ' . openssl_error_string());
             }
@@ -125,7 +125,7 @@ public function decrypt(AbstractKey $key, string $ciphertext): string
                 $fn = 'openssl_private_decrypt';
             }
 
-            $plaintext = null;
+            $plaintext = '';
             if (!$fn($ciphertext, $plaintext, $key->get(), $this->padding)) {
                 throw new RuntimeException('Cannot decrypt data: ' . openssl_error_string());
             }